You have the shot all set up, composition just right, and all that great light is coming in through the lens... *click*. Then you look at the preview, and what's this? There's a big warped square of brightness in the shot! Who made that thing?
You line up all the likely suspects against the wall. Who looks the most guilty...yes, it's the Sun!
Bright sunlight striking the lens, even from a crazy angle, can cause all kinds of lens flare problems, from washing out an image down to your classic multi-element lens flare effect.
If the sun's in shot, there's really not much you can do to improve the state of affairs except to reposition the camera or wait until it's out of shot.
But even if the sun is out of shot, sometimes its pesky light (I know, it's a love-hate relationship) can still directly hit the lens and cause problems.
To combat this, you'll often see photographers with big hoods on their lenses that act as shades to keep the sunlight out. This is very effective, but they probably don't sell hoods for your cameraphone. Fortunately, most people have an additional "hood" on their persons at all times: their other hand! Just hold you hand in such a way that the lens falls under its shadow. Usually it works to cup your hand around the lens like a megaphone horn.
How can you tell when the lens is in shadow? You can actually see it in the preview—the image will look less washed out when the shadow is over the lens.
Make sure your hand isn't visible in-shot!